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Monday, September 24, 2012

HF Monitoring at Sweeney's Flats, Eltham

I was out at Sweeney’s Flats, Eltham, yesterday, on a 4 km bushland hike alongside the Yarra, about 20 km east of central Melbourne.

I had a look at propagation in the window 0530 to 0600, using the Eton E5. The antenna was small – three metres hooked on to a wooden fence post, about 1 m above the ground, with good signals on 11 MHz, 13 MHz, 9 MHz, 15 MHz and 17 MHz The area is relatively free of power line interference.

In my opinion, there is no technical benefit in using very long antennas for HF monitoring. There is a myth within the HF monitoring community that the longer the antenna, the better the reception, which has spawned the idea that 200 metres (or greater!) long wires will yield marvellous results. There is also a myth that such antennas will display directional properties, if “pointed” to specific continents.

In suburban areas, the noise floor can be very high, which includes the component of mains-generated radiated interference, The best HF antenna is a simple half-wave dipole with a balanced feeder, using a Pi-section coupler/tuning unit, which yields an improved S/N ratio broadband, which knocks out a lot of that mains-generated noise.

My own antenna is a simple 5 MHz dipole, about 30 metres long, with each half about 15 metres, connected to the tuning unit with ordinary plastic covered power flex, which has a characteristic impedance of about 50 ohms, near enough to that at the centre point of a half-wide simple dipole. If there is not enough space for a 5 MHz dipole, shorter dipoles will work almost as well - a 9 MHz dipole is only about 15 m total length.

Failing that, use a vertical wire, as high as possible, with a simple single wire connection to the receiver, which will work fine! If in doubt, hang your vertical antenna from a balloon or kite, as Marconi did!

Connect the output of the Tuning Unit to the antenna and earth terminals – impedance matching is not ideal, but good enough as the antenna does not carry power. If you know what you are doing, you can connect the TU to the receiver via a Balanced-to-Unbalanced (BALUN) circuit, as the Output Z of the TU is around 300 Ohms, and simple receivers are around 50 Ohms.

Here is a summary of selected entries made at Sweeney’s Flats on September 23, 2012, including some excellent Brazilian signals:

9565 BRAZIL R. Tupi
9820 BRAZIL R. 9 Julho
11765 BRAZIL R. Tupi
11780 BRAZIL R. Nacional
11815 BRAZIL R . Brazil Central
13610 CHINA CNR1
13660 CYPRUS BBC Arabic
13695 FRANCE RFI-Issoudin French
13700 CHINA CNR1
13710 IRAN Turkish *0530
13740 IRAN Dari
13765 MADAGASCAR VR- Talata Portuguese
13785 IRAN Turkish *0530

17485 CHINA CRI-Kashi Arabic
17505 CHINA CRI-Kashi English
17530 IRAN Spanish *0530
17550 IRAN Arabic
17560 THAILAND RL-Udon Russian
17565 CHINA CNR1
17580 CHINA CNR1
17595 CHINA CNR1
17605 CHINA CNR1
17615 MARIANAS RFA-Tinian Chinese
17625 CHINA CNR2
17660 IRAN Bosnian
17690 THAILAND R. Azadi, Pashto
17720 CHINA CRI-Kashi German
17730 CHINA CRI-Xian English
17740 CHINA CRI-Xian Vietnamese
17760 ROMANIA RRI English
17770 THAILAND R. Thailand, Udon English
17830 THAILAND VOA-Udon Tibetan
17840 SRI LANKA R. Farda, Iranawila, Farsi
17855 CHINA CRI-Beijing, English
17870 SRI LANKA VOA-Iranawila, Kurdish
17895 S. ARABIA BSKSA Arabic

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